Inkjet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image recording element to yield the desired image. In one process, known as continuous inkjet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand inkjet, individual ink droplets are projected as needed onto the image recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Inkjet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant that is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. The carrier medium can be a liquid or a solid at room temperature in both cases. Commonly used carrier media include water, mixtures of water and organic solvents and high boiling organic solvents, such as hydrocarbons, esters, and ketones. Aqueous-based ink compositions are preferred because they are more environmentally friendly as compared to solvent-based inks, plus most print heads are designed for use with aqueous-based inks.
Materials used in inkjet printing inks must have the correct properties to provide an ink which is stable, possesses good printing properties and provides an image with good color, sharpness and image stability. The ink composition may be colored with pigments, dyes, polymeric dyes, loaded-dye/latex particles, or any other types of colorants, or combinations thereof. The ink composition may be yellow, magenta, cyan, black, gray, red, violet, blue, green, orange, and brown. Many colorants are known and used in inkjet printing inks. Many have some or most of these desirable properties but it is very difficult to find a colorant which possesses all of the above attributes. Inkjet inks generally contain a dye that is soluble in an ink vehicle such as water or a mixture composed of water and a known water soluble or water miscible organic solvent. Water solubility of these dyes is due to the incorporation of negatively charged substituent groups such as sulfo or carboxy. Colorants are degraded by ozone and light and their stability with regard to these two agents can differ depending on media and ink composition. There is a great need to develop dye-based inks that have high optical densities on receivers and also superior light fastness and colorfastness when printed on different types of media, in particular, fast drying or porous media as well as plain paper.
The colorants currently in commercial aqueous ink formulations are less than optimal in one property or another. They are used because they achieve an acceptable but not superior balance of features. Some examples of such dyes are Direct yellow 132 (CAS 10114-86-0) and Direct yellow 86 (CAS 50295-42-3), acid yellow 23 (CAS 1934-21-0) and acid yellow 17 (CAS 6359-98-4) which have good hue and ozone fastness, but poor light stability. U.S. Pat. Nos. 6,468,338 and 6,464,767, incorporated herein by reference, disclose water-soluble azoindole dyes for use in inkjet printing including dyes derived from diazotizable heteroaromatic amines. Dyes suitable for use in inkjet inks include those commonly used in the art of inkjet printing. For aqueous-based ink compositions, such dyes include water-soluble reactive dyes, direct dyes, anionic dyes, cationic dyes, acid dyes, food dyes, metal-complex dyes, phthalocyanine dyes, antaraquinone dyes, anthrapyridone dyes, azo dyes, rhodamine dyes, solvent dyes and the like. Some other water soluble dyes include Acid Red 52; azo-naphthol dyes, such as Acid Red 249, Direct Red 227 or Reactive Red 31; azo-aniline dyes; azo-thiazole dyes; arylazonaphthylamine dyes, such as Direct Red 75 or Ilford M-377; metal-complex azo dyes, such as Reactive Red 23, CAS Registry No. 347417-99-6 or any of the dyes described in U.S. Pat. No. 6,001,161 and anthrapyridone dyes, such as Acid Red 80 and 82
While glossy, porous media have the ability to absorb high concentrations of ink instantly, they suffer from image fastness problems, such as fading due to exposure to radiation by daylight, tungsten light, fluorescent light, or ozone, as described by D. E. Bugner and C. Suminski, “Filtration and Reciprocity Effects on the Fade Rate of Inkjet Photographic Prints”, Proceedings of IS&T's NIP16: International Conference on Digital Printing Technologies, Vancouver, BC, October 2000. It is believed that the poor image fastness may be attributed to the greater permeability of the porous image receiving layers (IRL's) to oxygen and/other airborne reactants such as ozone. Porous media is important because such fast drying media is becoming more and more popular and while most colorants are stable on gel-based or swellable media, porous media are more stringent in their demand for a high level of stability. It is also important for colorants to perform well on all grades of plain paper. Plain paper is used by most consumers in home inkjet printers.
Pigments are used in inkjet printing in an effort to solve some of the problems present with dye-based inks. Pigments generally have better stability to light and ozone than dyes. The physical properties of the pigment present a new group of challenges. The pigment particles must be very small in order to give good transparency and high density when printed. This is often achieved by grinding or milling the pigment in the presence of a suitable dispersant. The pigment must have very low solubility in the ink formulation and must not be prone to clumping or particle growth, or poor jetting behavior will result. It is often hard to tailor the hue of a pigment to that which is desired because the physical properties of the intermolecular array, formed by the interaction of the individual pigment molecules with each other, often overwhelm the electronic effect of the substituents on the individual molecules. Hue is also affected by crystal morphology and particle size distribution. It is often hard to control these features without extensive trial and error experimentation with each pigment to find the preparation conditions and or the post treatment that imparts the desired property. A wide variety of organic and inorganic pigments, alone or in combination with additional pigments or dyes, may be used in the ink composition of the present invention. Pigments that may be used in the invention include those disclosed in, for example, U.S. Pat. Nos. 5,026,427; 5,086,698; 5,141,556; 5,160,370; and 5,169,436, all incorporated herein by reference. The exact choice of pigments will depend upon the specific application and performance requirements such as color reproduction and image stability.
Pigments suitable for use in inkjet inks include, but are not limited to, azo pigments, monoazo pigments, disazo pigments, azo pigment lakes, β-Naphtho pigments, Naphthol AS pigments, benzimidazolone pigments, disazo condensation pigments, metal complex pigments, isoindolinone and isoindoline pigments, phthalocyanine pigments, quinacridone pigments, perylene and perinone pigments, thioindigo pigments, anthrapyrimidone pigments, flavanthrone pigments, anthanthrone pigments, dioxazine pigments, triarylcarbonium pigments, quinophthalone pigments, diketopyrrolo pyrrole pigments, titanium oxide, iron oxide, and carbon black. Although many classes of pigments are known, the producers of inkjet inks tend to limit their pigment choices to a relatively small subset of pigments that are commercially available at high purity and that behave well in the ink formulation.
GB988972 discloses fabric dyes comprising aromatic azo dyes bearing an aryl group with an ortho substituent capable of complexing to a metal atom. GB988972 does not disclose aromatic azo dyes incapable of forming a metal complex at that substituent site or which are heterocyclic and which are also useful in inks.